Robert Shiller's cyclically adjusted price-earnings (CAPE) ratio for the S&P 500, which includes 10 years of earnings, has soared to 27.2, a level topped only by the pre-crash periods of 1929, 2000 and 2007.

Yet the stock market continues its six-year rally unbowed, with the S&P 500 standing less than 3 percent from its record high. So what does Nobel laureate economist Shiller make of it all?

"This stock market is an enigma," he told MarketWatch. "Short-run forecasts are very difficult," but the CAPE ratio signals trouble. "When CAPE is this high, 10-year returns on the S&P 500 are nearly flat, because inevitably there is a major correction," Shiller said.

The S&P 500 hasn't endured a 10 percent correction since October 2011, although it almost reached that magnitude last September-October.

To be sure, "while history suggests current levels signal a correction, it does not mean we will have one soon," Shiller said. "CAPE could go higher, as it did in 2000, so until then, stocks could keep rising. The problem is, you cannot time [a correction] precisely."

Carter Worth, head of technical analysis at Cornerstone Macro, also sees signs of danger for stocks.

The problem is that major sectors of the index have failed to reach new peaks, he told CNBC. "The issue is trying to come to a judgment not so much by looking at the market, but by looking at the parts that comprise the whole," he explained.

"So while the chart of the S&P might say one thing, the other parts are giving a different message and that's been the case for the better part of the past 10 to 12 months."

Volatility is another issue. The CBOE Volatility Index (VIX), has jumped 13 percent since the S&P 500 hit its record high March 2.

"I would characterize this as a high-volatility, low-variance moment, meaning we have huge daily swings and yet we have no variance. That kind of churn is usually a problem."

So where are we headed from here? "I think the best thing we are hoping for is a '98 type outcome," Worth said. The S&P 500 fell 18 percent from July 1998 to September 1998. "We saw a huge downdraft and then we recovered. We need a reset."

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